In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Conventional methods of measuring resistance in resistive elements typically use some form of RMS or other time-averaging schemes or apply an active signal to the restive elements in order to perform a measurement. In the case of the former, the measurements are usually heavily filtered to remove noise and therefore do not provide the resolution or tracking speed that the method of this invention provides. Furthermore, these measurements tend to not be very accurate with time-proportioned (such as in the case of zero-cross synchronous control) control methods. In the case of the latter, the measurements often need to be taken when the power is not applied (which is in direct contrast to in-situ measurements made when power is applied under normal operating conditions) and can be a source of system noise depending on the signal applied to enable the measurement. In cases where the power supply is DC, it may not be desirable or possible to interrupt the voltage and current supply to take a measurement.
A method for monitoring the condition of an electric current-carrying heating element is disclosed in U.S. Pat. Nos. 6,956,489 and 7,012,538, the contents of which are incorporated herein by reference.